One type of rolling element bearing assembly used in mechanical applications is a hard mount assembly. Hard mount assemblies differ from other assemblies in that the rolling element bearing is interposed between a rotatable shaft and stationary structure without a squeeze film or spring cage damper. A standard bearing is comprised of a plurality of rolling elements confined to a raceway defined by an inner ring and an outer ring. In a standard hard mount assembly, the bearing's inner ring is mounted to the rotatable shaft so that the inner ring and shaft rotate together during engine operation and the bearing's outer ring is slip fit mounted to the stationary structure so that the top surface of the bearing is encircled by the inner surface of the structure. The outer ring is frictionally secured to the stationary housing through mechanical loads which usually create a normal force on one side of the bearing. Frictionally engaging the outer ring to the stationary structure through an interference fit will create a high compressive load on the rolling elements and will impede the rotation thereof.
In a hard mount bearing assembly, the bearing is usually made of bearing quality steel, the rotatable shaft is usually made of steel, and the stationary structure is often made of lower strength material to reduce weight.
The purpose of the rolling element bearings is to position rotatable shafts in the stationary structure and transfer the radial loads from the shaft to the structure while permitting relatively low friction rotation of the shaft.
A common disadvantage to hard mount bearing assemblies is that uncoupling of the frictional engagement between the outer ring and the stationary structure can create significant rubbing and wear resulting in a misaligned bearing mount. In a hard mount assembly, the bearing mounting is the shaft and the stationary structure race, both of which should be cylindrical as opposed to being coned or out-of-round. Noncylindrical surfaces results in the inner and outer rings not forming a circular race causing excessive rubbing of the rolling elements against the race which shortens bearing life.
Misalignment from bearing outer ring rub is particularly evident for a stationary structure of a material which has a greater coefficient of thermal expansion and is softer than bearing steel, such as aluminum. During mechanical operation, the temperatures of the shaft, the bearing and the stationary structure will often increase causing the stationary structure race to expand more than the bearing. As the components thermally grow apart, the frictionally engaged area between the bearing outer ring and the structure is decreased resulting in the bearing outer ring rotating within the structure due to circumferential forces and vibration exerted from the shaft. The harder, bearing outer ring will wear the softer, stationary structure creating debris and a misaligned bearing mount. The debris will often embed itself into the rolling element bearing inhibiting the bearing rotation and the misaligned mount causes additional bearing wear as explained above.
The common prior art has been to press fit the bearing outer ring into the stationary structure and press fit the inner race to the rotatable shaft or to insert an anti-rotation pin between the bearing outer ring and the stationary housing. As discussed above, press fitting is very disadvantageous because of the large compressive forces that clamp the rolling elements resulting in higher rolling friction and a very short bearing life. Anti-rotation pins have proved to be expensive and difficult to incorporate. U.S. Pat. No. 4,195,947 discloses a method of overcoming outer ring rotation in compacting rollers by securing the inner ring and outer ring by a plurality of threaded screws. This method of anti-rotating is disadvantageous because it increases the number of components significantly and increases the space required for the bearing.
Accordingly, a need exists for rolling element bearing assemblies that resist the circumferential forces and vibration exerted by the rotating shaft to prevent bearing misalignment and the creation of debris.